Circuit arrangement

ABSTRACT

A circuit arrangement having a plurality of variable capacitance elements such as varactors is described, the varactors having associated electronic control means which controls the capacitance of the variable capacitance elements over a control range. The control range is such that for any particular variable capacitance element a complete variation from a lowest to a highest capacitance is obtained from only a portion of the control range.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to currently pending UnitedKingdom Patent Application number 0327284.6, filed Nov. 24, 2003.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

BACKGROUND OF THE INVENTION

Varactors are commonly used in RF circuits for tuning oscillators,filters and amplifiers.

One problem with varactors is that their capacitance/voltagecharacteristics are typically very non-linear as shown in FIG. 1 a,which illustrates a typical metal oxide semiconductor varactor (MOSvar)capacitance/voltage characteristic. The non-linear feature of the MOSvaris emphasized by FIG. 1 b which shows the first derivative dC/dV of thecurve of FIG. 1 a.

One device allowing a capacitance/voltage characteristic having anacceptable tuning range and a more linear range to be obtained is ahyper-abrupt varactor. However, the implementation of a hyper-abruptvaractor requires extra processing during manufacture, which isexpensive.

An alternative method of overcoming the non-linearity of a varactor isto use digital techniques to switch in capacitors so as to tune over therequired range. However, this solution is complex, can be physicallylarge, and may be too slow.

It is an object of the present invention to provide a variablecapacitance circuit arrangement having a relatively linearcharacteristic.

OBJECTS AND SUMMARY OF THE INVENTION

Additional objects and advantages of the invention will be set forth inpart in the description that follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

According to a first aspect of the present invention, there is provideda circuit arrangement having a variable capacitance for a tuningcircuit, wherein the circuit arrangement comprises a plurality ofvariable capacitance elements connected in parallel and, coupled to thecapacitance elements, control means for electronically controlling thecapacitances of the variable capacitance elements, the control meanshaving a control range over which they cause the capacitance of thecircuit arrangement to vary, the control means and the variablecapacitance elements being configured such that at least one of the saidelements exhibits complete variation of its capacitance in response tothe control means over only a portion of the control range.

The characteristics of the capacitance elements determine the way inwhich the capacitance of the circuit arrangement varies in response tothe control means. For example, the variable capacitance elements may bechosen so as to produce a capacitance versus control voltage responsewhich is more linear over its operating range compared with that of asingle varactor. Similarly, the variable capacitance elements may bechosen so as to produce a capacitance response which follows anapproximate square law characteristic for a linearized frequency/voltagecharacteristic when used in conjunction with an inductor to form aresonant network.

The control means may comprise a common control source and a pluralityof different respective offset biases applied to the variable capacitiveelements.

In a preferred embodiment, the variable capacitive elements are varactor(varactors), which are biased by an offset voltage. In this embodiment,the control means is configured to apply a common control voltage and aplurality of different DC offset voltages to the respective varactors.The voltage applied across each varactor is, therefore, the differencebetween the common control voltage and the respective offset voltageapplied to each varactor, (or the sum of these two voltages, dependingon the sign of the offset voltage). When tuning, the varactor onlyexhibits a change in its capacitance if the difference between thecontrol voltage and the respective offset voltage applied to theparticular varactor falls within the range of voltages over which thevaractor capacitance varies, in terms of the voltage applied across thevaractor itself. Alternatively, the control means may be configured toapply a plurality of different control voltages to the respectivevaractors. A common bias voltage may be applied.

Each varactor may be arranged to have one of its electrodes coupled tothe common control voltage source and its other electrode to arespective DC offset bias voltage source.

The capacitance characteristic of the circuit arrangement is dependenton the number of variable capacitive elements connected in parallel. Themore variable capacitive elements used in the circuit, the closer thecapacitance characteristic can be to a desired response i.e., linear,square law, etc.

Advantageously, the control means are arranged such that the variableparts of the capacitance response characteristics of the variablecapacitive elements overlap. By adjustment of the overlaps, it ispossible to alter the overall characteristic of the circuit arrangementto be closer to the desired characteristic.

The variable capacitance elements are preferably selected such that thesum of their individual capacitance values is equal to the requiredtotal maximum capacitance of the circuit arrangement. Additionally, thevaractors may be selected such that the combination of the ranges overwhich their individual capacitances vary is substantially equal to thetotal operational range of the circuit arrangement,

In one embodiment, where a generally linear capacitance/voltagecharacteristic is required, the individual variable capacitance elementsmay be chosen such that the maximum capacitance of each element isapproximately equal to the maximum required capacitance of the circuitarrangement divided by the number of parallel variable capacitiveelements. In addition, the individual variable capacitive elements maybe chosen such that the range over which each of their individualcapacitances vary is equal to the total operating range of the circuitarrangement divided by the number of capacitive elements connected inparallel. For example, if there are three variable capacitive elementsin the circuit, the characteristics of the capacitive elements are suchthat their individual maximum capacitances are each equal to a third ofthe total capacitance of the circuit arrangement and their effectiveoperating ranges are each approximately a third of the total operatingrange of the arrangement.

In an alternative embodiment the characteristics of the capacitanceelements may be selected such that the resultant capacitance/voltagecharacteristic has a square law characteristic. In this embodiment thecharacteristic of each capacitance element may be different from thecharacteristics of the other capacitance elements, unlike thecharacteristic of capacitance elements that may be selected if agenerally linear CN characteristic is desired.

According to a second aspect of the present invention, there is provideda tunable radio frequency (RF) circuit comprising a circuit arrangementhaving a variable tuning capacitance, wherein the circuit arrangementcomprises a plurality of tuning varactors connected in parallel, andcoupled to the tuning varactors, control means for electronicallycontrolling the capacitances of the varactors, and one or moreinductors, the control means having a control range over which theycause the capacitance of the circuit arrangement to vary, the controlmeans and the varactors being configured such that at least one of thevaractors exhibits complete variation of its capacitance in response tothe control means over only a portion of the control range.

In one embodiment, the circuit includes a modulator which comprises amodulation varactor arranged in parallel with the tuning varactors butisolated therefrom by a DC blocking capacitor, the modulation varactorbeing coupled to a modulation input. The modulation varactor may have anoffset voltage applied to one of its electrodes.

According to another aspect of the present invention, there is provideda voltage controllable oscillator comprising a circuit arrangementhaving a variable capacitance for a tuning circuit, wherein the circuitarrangement comprises a plurality of varactors connected in parallel,and coupled to the varactors, control means for electronicallycontrolling the capacitances of the varactors, the control means havinga control range over which they cause the capacitance of the circuitarrangement to vary, the control means and the varactors beingconfigured such that at least one of the varactors exhibits completevariation of its capacitance in response to the control means over onlya portion of the control range.

The varactors in the circuit arrangement may be selected such that thecapacitance/voltage characteristic of the circuit arrangement has agenerally square law characteristic so as to achieve a linearizedfrequency/voltage response characteristic for the voltage controllableoscillator.

In one embodiment, the voltage controllable oscillator includes amodulator as described hereinabove. This arrangement provides theoscillator with a tuning circuit wherein the frequency of the oscillatorcan be modulated by a modulation signal independently of the mainfrequency control signal.

According to yet another aspect of the present invention, there isprovided a tunable radio frequency (RE) circuit comprising: the resonantcombination of a tuning varactor, a modulation varactor and anassociated inductance, the tuning varactor and the modulation varactorbeing respectively coupled to a tuning control input and modulationinput which are DC isolated from each other.

The invention also includes a tunable RF circuit including a pluralityof varactors connected in parallel with each other and, coupled to thevaractors, capacitance control means operable to apply differentvariable voltages simultaneously across respective varactors.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate at least one presently preferredembodiment of the invention as well as some alternative embodiments.These drawings, together with the description, serve to explain theprinciples of the invention but by no means are intended to beexhaustive of all of the possible manifestations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a graph illustrating a typical capacitance/voltage (CN)characteristic of a varactor;

FIG. 1 b is a graph illustrating the first derivative (dC/dV) of thecharacteristic of FIG. 1 a;

FIG. 2 is a schematic diagram of a circuit arrangement in accordancewith the present invention;

FIG. 3 is a graph illustrating a CN characteristic of the circuitarrangement shown in FIG. 2;

FIG. 4 is a graph illustrating the CN characteristics shown in FIGS. 1 aand 3 in a single representation;

FIG. 5 is a graph illustrating the first derivative (dC/dV) of thecapacitance response of the circuit arrangement of FIG. 2;

FIG. 6 is a schematic diagram of a circuit arrangement in accordancewith the invention, including a modulator; and

FIG. 7 is a schematic diagram of a voltage controlled oscillatorincluding a circuit arrangement in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference now will be made in detail to the presently preferredembodiments of the invention, one or more examples of which areillustrated in the accompanying drawings. Each example is provided byway of explanation of the invention, which is not restricted to thespecifics of the examples. In fact, it will be apparent to those skilledin the art that various modifications and variations can be made in thepresent invention without departing from the scope or spirit of theinvention. For instance, features illustrated or described as part ofone embodiment, can be used on another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncover such modifications and variations as come within the scope of theappended claims and their equivalents. The same numerals are assigned tothe same components throughout the drawings and description.

Referring to FIG. 2, a variable capacitance circuit arrangement 100 hasa variable capacitance formed by three varactors 110, 112, 114 which areconnected in parallel. The varactors 110, 112, 114 are each connected atone of their electrodes to a respective DC offset voltage source V1, V2,V3 and, at the other electrode, to a common control input 116 forsupplying a control voltage V_(control) via a series impedance 118. Inthis embodiment each of the offset voltage sources V1, V2, V3 isconnected in series between the anode of the respective varactor and oneof the supply rails of the arrangement, in this case to ground, thevaractors cathodes being coupled to the control input. The DC offsetvoltage sources V1, V2, V3 each have a different offset voltage levelsuch that each of the varactors 110, 112, 114 contributes to thevariation in overall capacitance only when the difference between thecontrol and the respective offset bias falls within the voltage range(in terms of the voltage across the varactor) over which the varactorexhibits a variation in capacitance.

In this embodiment, the different offset voltages V1, V2, and V3 aresuch that V1 is smaller than V2 which is, in turn, smaller than V3.Therefore, assuming that the individual characteristics of the varactorsare similar, if the control voltage is raised progressively from thelower limit of its range to its upper limit, the operation of thecircuit 100 will sequentially bring the first varactor 110 into thevariable part of its characteristic, followed by the second varactor112, and finally the third varactor 114.

The different offset voltage levels may be achieved by a number ofdifferent arrangements, as would be known by a person of ordinary skillin the art. These arrangements can include the use of a voltage dividercircuit, zener diodes, individual DC power sources and the like.

The characteristics of the varactors 110, 112, 114 are selected suchthat the sum of their maximum respective capacitances is equal to therequired total maximum capacitance of the circuit arrangement 100. Inaddition, the varactor characteristics are selected such that the sum ofthe maximum ranges of variation in capacitance of the varactors is equalto the required total variation in capacitance of the circuitarrangement 100. If a substantially linear variation of the overall,capacitance with control voltage is required, the varactors are selectedso as to have the same or generally similar characteristics, at leastinsofar as they have at least approximately equal capacitance ranges andcapacitance-versus-voltage slopes. If the overall capacitance is tofollow an approximate square law characteristic with respect to voltage,as may be required to achieve a linearized tuningfrequency-versus-voltage characteristic in a resonantinductance/capacitance circuit such as in an RF voltage-controlledoscillator (VCO), the varactors 110, 112, 114 may be selected to havedifferent capacitance ranges. For instance, the varactor associated withthe highest offset voltage may be chosen to have a greater range ofvariation of capacitance and a steeper capacitance-versus-voltage slope.

The offset bias voltages applied to the varactors are preferably setsuch that there is an overlap, with respect to control voltage, betweenthe high capacitance part of the variable capacitance range of onevaractor and the low capacitance part of the range of capacitance ofanother of the varactors. Overlapping of the variable portions ofrespective characteristics in this way, as depicted in FIG. 3,contributes to the linearity of the capacitance/voltage characteristicsof the composite arrangement 100.

The circuit arrangement 100 is operable such that as the control voltageis increased from a minimum to a maximum voltage, each of the varactorsis sequentially operated. That is to say, as the control voltage isincreased the varactors are activated such that there is an overlapbetween the high capacitance range of one varactor and the lowcapacitance part of the range of capacitance variation of another of thevaractors. The total capacitance of the circuit arrangement 100 isequivalent to the sum of the capacitance of each of the varactors.

FIG. 4 shows the capacitance versus voltage characteristic of thecircuit arrangement 100 of FIG. 2 superimposed on the equivalent curveof a circuit having a typical single MOSvar, as shown in FIG. 1 a. Itcan be seen that the curve of the circuit arrangement 100 is more linearthan that of the MOSvar. This fact is more clearly seen in FIG. 5 whichillustrates the first derivative dC/dV of the characteristic of FIG. 3.It can be see that the circuit arrangement 100 produces less variationin the dC/dV characteristic over the operational control voltage rangecompared with that of the dC/dV characteristic of the typical MOSvar, asevidenced by the approximately flat character of the relevant part ofthe curve.

To summarize, the circuit arrangement 100 has a capacitive network madeup of a number of varactors connected in parallel, each varactor beinginherently non-linear over its operating range, and yet the network as awhole having the advantage of a more linear capacitance versus controlinput response compared to that of the typical varactor implementationcapable of capacitance variation over the same range.

The circuit arrangement 100 has many different applications. However, itis of particular benefit in RF tuning circuits such as voltagecontrolled oscillators, filters and tuned amplifiers.

Referring now to FIG. 6 of the drawings, the capacitance part of avoltage controlled oscillator 200 in accordance with the inventionincludes a modulator 220. Tuning of the oscillator is accomplished by anetwork of parallel varactors coupled to a control input and respectiveoffset sources as described above with reference to FIG. 2. Themodulator 220 comprises a varactor 222 connected effectively in parallelwith the varactors 110, 112, 114 of the tuning network. The modulatorvaractor is connected at one of its electrodes to a DC offset biasvoltage source V4 and at its other electrode to a modulation input 223for receiving a modulation signal V_(mod). The varactor 222 is coupledto the circuit arrangement 100 via a DC blocking capacitor 224, therebyisolating the modulation input 223 from the control input 116.

Use of an additional varactor 222 specifically for frequency modulationof the VCO output signal, the modulation being applied to this varactordirectly from a modulation input which is isolated from the controlinput 116, has the advantage that the sensitivity of the modulationprocess can be set substantially independently of the VCO tuningfrequency. That is to say, the variations in capacitance produced by themodulation signal applied to the modulation input 226 do not varysignificantly in magnitude for a given modulation voltage amplitude asthe VCO operating frequency alters. Accordingly, the depth of modulationremains substantially constant.

Referring to FIG. 7, an emitter coupled LC oscillator 300 in accordancewith the invention has a cross-coupled transistor pair Q0, Q1 arrangedas a voltage controllable oscillator with a differential output acrossthe collectors of the transistors Q0, Q1.

The frequency of the oscillator 300 is determined by the inductive andcapacitive components connected to the collectors of the cross-coupledtransistor pair Q0, Q1 and the virtual ground formed by a bias block 302which incorporates a plurality of offset voltage sources producingvaractor bias voltages V1, V2 and V3. In this circuit, thefrequency-determining components are inductors L1 and L2, capacitors C0and C1 and varactors C10, C11, C12, C13, C14 and C15.

Each varactor is connected to a respective DC offset voltage source V1,V2 or V3 in the bias block 302 and the total capacitance of thevaractors is adjusted by varying the value of the control voltage,V_(control).

Accordingly, the connections between the voltage bias block 302 and thevaractors connected to bias voltages sources V1, V2 and V3 can beconsidered to be an RF ground. Therefore, the varactors C10, C1 and C13,located on the left hand side (LHS) of the circuit, are effectivelyconnected in parallel at radio frequencies. The total capacitance of thefrequency-determining components on the LHS of the circuit is thecapacitance resulting from the connection of capacitor C0 in series withthe total capacitance of the parallel-connected varactors C10, C1 andC13. Similarly, the capacitance of the frequency-determining componentson the right-hand side of the circuit comprises capacitor C0 in serieswith the parallel combination of the varactors C13, C14 and C15. Thetotal capacitance of the frequency-determining components in theoscillator 300 is equal to the overall capacitance of the frequencydetermining capacitances (C1, C13, C14, C15) on the RHS in series withthe overall capacitance of the frequency determining capacitances (C0,C10, C11, C12) on the LHS.

The total inductance of the inductive frequency-determining componentsin the oscillator 300 is equal to the inductance of inductor L1 inseries with that of the inductor L2.

The transistors Q0, Q1 are connected at their bases to a bias voltagesource VB via resistors R3 and R4 respectively so as to forward biastheir base-emitter junctions.

The transistors Q0, Q1 are capacitively cross-coupled. Specifically,coupling capacitors C2 and C3 couple the signals generated at thecollectors of transistors Q1 and Q0 to the bases of the transistors Q0,Q1 respectively to cause oscillation in a well-known manner. Thevaractor pairs C10, C 13; C10, C14; and C12, C15 are selected such thatthe varactors of each pair have the same CN characteristic. However, theCN characteristic of each pair may be selected to have a differentcharacteristic and in particular different capacitance ranges. In apreferred embodiment, the CN characteristic of the complete set ofvaractors follows a square law curve in order to achieve a linearizedfrequency/voltage characteristic for the voltage controllableoscillator. This can be achieved, for example, by use of a varactorassociated with the highest offset voltage which has a characteristichaving a steeper CN curve and extends over a larger capacitive range.

Variations may be made without departing from the scope of theinvention. For example, the control means may comprise a plurality ofcontrol sources connected to the plurality of variable capacitanceelements; or a common offset bias and a plurality of different valuecontrol sources connected to the capacitance elements. Furthermore, thecircuit arrangement 100 may be used for a tunable filter or any otherapplication requiring a linearized variable capacitance.

A presently preferred embodiment of the subject invention is shown inFIG. 2.

While at least one presently preferred embodiment of the invention hasbeen described using specific terms, such description is forillustrative purposes only, and it is to be understood that changes andvariations may be made without departing from the spirit or scope of thefollowing claims.

1-17. (canceled)
 18. A tunable radio frequency (RF) circuit comprising:a resonant combination of a tuning varactor, a modulation varactor andan associated inductance; and a tuning control input and modulationinput coupled to the tuning varactor and the modulation varactor, thetuning control input and modulation input being DC isolated from eachother.
 19. A tunable RE circuit according to claim 18, wherein thetuning varactor is provided with a DC voltage offset.
 20. A tunableradio frequency (RF) circuit comprising: a circuit arrangement having: aplurality of tuning varactors effectively connected in parallel toprovide a variable tuning capacitance; at least one inductor; andcontrol means, coupled to the tuning varactors, for electronicallycontrolling the capacitances of the varactors, having a control rangeover which the varactors cause the capacitance of the circuitarrangement to vary, wherein the control means and the varactors areconfigured such that at least one of the varactors exhibits completevariation of its capacitance in response to the control means over onlya portion of the control range; and a modulator having: a modulationvaractor arranged in parallel with the tuning varactors but isolatedtherefrom by a DC blocking capacitor; and a modulation input coupled tothe modulation varactor.
 21. A tunable RF circuit arrangement accordingto claim 20, wherein the modulation varactor has a voltage offsetapplied to one of its electrodes.
 22. A tunable RF circuit according toclaim 20, wherein the control means comprises a common voltage sourceand a plurality of different respective offset voltage sources appliedacross each of the varactors.
 23. A voltage controllable oscillatorcomprising: a circuit arrangement having: a plurality of varactorseffectively coupled in parallel to provide a variable capacitance; andcontrol means, coupled to the varactors, for electronically controllingthe capacitances of the varactors, having a control range over which thevaractors cause the capacitance of the circuit arrangement to vary,wherein the control means and the varactors are configured such that atleast one of the varactors exhibits complete variation of itscapacitance in response to the control means over only a portion of thecontrol range; and a modulator which having: a modulation varactorarranged in parallel with the tuning varactors but isolated therefrom bya DC blocking capacitor; and a modulation input coupled to themodulation varactor.
 24. A voltage controllable oscillator according toclaim 23, wherein the varactors are selected so as to achieve a variablecapacitance having a generally square law characteristic with respect tovoltage such that a frequency/voltage characteristic of the oscillatoris linearised.